The present invention relates to a method and apparatus for continuously sampling plasma.
To analyze the various chemical properties of constituents contained in the blood, it is necessary that a volume of blood be extracted from the patient. In conventional practice, the whole blood is separated by centrifugation into cellular matter and plasma or serum. Chemical analysis is then performed on plasma or serum while the blood cells are discarded. Thus, repeated chemical analysis is associated with considerable loss of whole blood and blood cells. Frequent blood collections for diagnostic chemical analysis may cause anemia in severely ill, hospitalized patients, thereby worsening the clinical condition.
The prior art discloses several methods by which blood can be withdrawn and returned to the body. The blood can be withdrawn from an artery and returned through a vein by means of two separate surgical implantations. Dual venipuncture devices also require two separate needles, the second one (the outflow needle) being inserted into a different vein downstream, and placed in the same direction as the flow of blood. The duplex needle of U.S. Pat. No. 2,137,132 has a dual tubular construction, but the two tubes merge within the needle. Blood cannot be withdrawn and returned through the same needle. Instead, the flow of blood described in U.S. Pat. No. 2,137,132 requires two separate needles.
The dual flow cannula set of Consalvo U.S. Pat No. 4,098,275 has entry and exit apertures spaced substantially apart (1.1 inch) to avoid mixing of toxic blood components during hemodialysis of patients with renal failure. The 1.1 inch-distance between the apertures of the cannula requires positioning of the blood-intake aperture on the side of the cannula. This, in turn, necessitates shielding of the intake aperture to prevent collapse of the adjacent blood vessel wall over the aperture with subsequent blockage of blood flow upon attempts to withdraw blood. The high volume blood flow during hemodialysis is accommodated by a #15 gauge (0.072 in. 0.D.) needle. Placing two #15 gauge needles side-by-side creates a cannula of substantial size which requires surgical exposure of a large blood vessel for placement. Finally, the rigid, metallic cannula poses a constant risk for vascular injury and penetration thereby making prolonged intravascular placement of the cannula impractical.
U.S. Pat. No. 3,848,592 discloses a blood pump system used in dialysis with a single hypodermic needle with a v-shaped neck used for withdrawing and returning blood to the patient. However, in the system, separate valving means are necessary for changing the direction of blood flow.
U.S. Pat No. 4,614,513 relates to a method and apparatus for removing immunoreactive substances from blood. The treatment requires complete physical separation of blood cells and plasma in separate compartments. Addition of an anticoagulant to the system is essential to prevent coagulation of the separated blood cells. Therefore, U.S. Pat. No. 4,614,513 is concerned with a blood treatment system which in operation would not be closed.
In other conventional procedures for removal of blood from the patient, the line usually contains a filter and pumping means and means for sampling the blood from the line. In this regard, the system must be opened for removal of the sample of blood so that the blood may be analyzed or that the plasma can be separated. An anticoagulant is usually added to the blood to prevent the blood from clotting.
Due to the problems associated with the use of anticoagulants or other agents, there is a need for the sampling of blood and, more particularly, plasma analysis for diagnostic purposes, wherein the system is closed. The closed system has less reliance on the use of anticoagulants because of the quick transit time of the blood through the system.